This invention relates to integrated circuit and microelectromechanical systems (MEMS) manufacturing. More particularly, this invention relates to a system and method for accessing features of interest on an encapsulated device.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be a cantilevered beam which connects one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable cantilevered beam switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.
Because the MEMS devices often have moveable components, such as the cantilevered beam, they typically require protection of the vulnerable moveable portions by sealing the devices fabricated on a device wafer with a protective cap or lid wafer, to form device wafer/lid wafer assembly containing a plurality of encapsulated MEMS devices. Furthermore, the MEMS devices may be intended to operate in a particular environment. For example, a MEMS switch handling high voltages may be required to operate in an electrically insulating environment, and thus the MEMS switch may be encapsulated with an electrically insulating gas. In order to prevent the preferred gas environment from leaking out over the lifetime of the switch, the environment may need to be sealed hermetically when the lid wafer and the device wafer are bonded into the wafer assembly.
Often, MEMS manufacturers prefer to ship whole wafer assemblies to customers before singulating the encapsulated die into the individual devices. Therefore, before dicing the MEMS wafer, it is often desirable to probe the MEMS wafer to test for functionality of the devices. This probing requires, in general, a set of pads coupled electrically to the encapsulated MEMS device, but located externally to the hermetic lid seal which encapsulates the device. Accordingly, access must be provided to those probe pads before the device wafer is diced.
FIG. 1 is a diagram of an exemplary encapsulated MEMS device 100 according to the prior art. The encapsulated MEMS device 100 may include a lid wafer 20 and a device wafer 30. At least one MEMS device 34 is fabricated on the device wafer 30. The fabrication methods for formation of the MEMS device 34 may include bulk machining or surface machining techniques, for example. At least one probe pad 32 is formed on the device wafer 30, and is coupled electrically to the MEMS device 34, although the circuitry which connects the probe pad 32 to the MEMS device 34 is not shown in FIG. 1. The probe pads 32 may be used to test the functionality of the MEMS device 34, before it is singulated. Because the probe pads 32 will be used for electrical access to the MEMS device 34, they are often made of a relatively soft metal, such as gold, and are therefore susceptible to damage.
The lid wafer 20 is attached to the device wafer 30 by an adhesive, to form a protective cap over the MEMS device 34. In order to allow the MEMS device 34 to move freely, a device cavity 24 may be formed in the lid wafer 20, by etching or other means of removing material, before it is adhered to the device wafer 30. Along with the device cavity 24, another cavity, the trench cavity 22, may be formed above the set of probe pads 32, to allow access to the probe pads by a probe device.
To provide access to the trench cavity 22 and underlying probe pads 32, the lid wafer 20 may be sawed above the trench cavity 22 by a narrow saw blade to remove the material 29 directly above the trench cavity 22 and probe pads 32. Typically, because the trench cavity is generally about 1 mm in width to allow access to the pads, and the saw blades are tens of microns wide, two saw cuts 28, shown in FIG. 1, may be required to remove the material 29 above the probe pads 32.
Several difficulties are associated with this prior art procedure. First, the saw blades may need to be carefully aligned with respect to the MEMS device, so that the saw cuts 28 are located directly above the trench cavity 22. This alignment is made more difficult because the trench cavity 22 cannot be seen directly from the top side of the wafer assembly. In addition, the saw cutting produces substantial debris, which may be flung by the saw blades over the probe pad 32 area. Furthermore, when the second saw cut is made, the lid remainder 29 is free from the surrounding material of the lid wafer 20. The lid remainder 29 therefore may fall down into the trench cavity 22 and into the vulnerable probe pad 32 area. This lid remainder 29 may prevent access to the probe pads 32, or short the probe pads 32, and interfere with the probing or the functioning of the MEMS device 34, and cause testing failures. Even if the presence of the lid remainder 29 on the probe pads 32 does not interfere with the functioning of the MEMS device 34, its presence may cause the parts to fail a visual inspection criterion. Such failures may negatively impact wafer yields in the most disadvantageous and costly point in the manufacturing process, after the device wafer is essentially complete.
Accordingly, a method is needed that provides access to the probe pads or other features of interest on the device wafer, while avoiding the difficulties associated with saw cutting of the lid wafer.